Ticks are the cause, directly or indirectly, of extensive damage to animals or humans. Examples of the direct damage are pruritus or bleeding caused by biting or blood-sucking, or tick paralysis or allergic diseases caused by saliva secreted in blood-sucking or regurgitation of midgut contents. Examples of the indirect damage are various diseases in livestock, caused by viruses, rickettsiae, bacterium, spirochaeta, protozoa, nematoda, or the like. This damage causes enormous losses at home and abroad, and threat of emerging and re-emerging zoonotic diseases by ticks is becoming a serious problem.
Under these circumstances, various methods to exterminate ticks are used in many countries. Among these methods, the major one is the use of agents such as organic phosphorus agents, carbamate agents, pyrethroid or macrolide antibiotics, or the like. However, in any agent, a drug resistance is established by using the agent successively or heavily, and thus many agents lose their miticidal activity. Further, when using such agents, it is necessary to take side effects to animals into consideration. In addition, there is a problem of a remnant agent which may threaten the safety of foods and the environment, and people tend to avoid the use of such agents. Furthermore, the use of agents is approaching limitation, with respect to the enormous development cost, in addition to the effectiveness thereof and an applicable area. As described above, it is considered difficult to prevent the parasitism of ticks to humans or animals, and the damage caused by ticks-borne infection in the 21st century, by means of the use of agents.
In hematophagous arthropods including ticks, acquisition of protective immune response against reinfection in a host against a viral or bacterial infection is known and has been confirmed in the laboratory stage [Fujisaki, Nat. Inst. Anim. Hlth. Quart. (Tokyo), 18, 27-38 (1978)]. Due to the recent progress in gene recombination techniques, genes encoding protective antigens, enzymes related to metamorphosis specific to hematophagous arthropods, or the like are being intensively cloned in many countries, and an attempt to manufacture safe vaccine proteins or chemotherapeutic agents has been made.
However, such an agent in practical use is only that against Boophilus microplus, which was developed by Willadesen [Willadesen and Jogejan, Prasitology Today. 15, 258-262 (1999)]. There is now a search for a vaccine against Haemaphysalis longicornis, which is widely distributed over Asian countries including Japan and the Eurasia continent and mediates zoonotic diseases such as piroplasmosis, Q fever, or viral encephalitis and thus the rapid development and practical application of such a vaccine is greatly desired.